Attach objects to every face of one model
Both Particles and Dupliframes have brought me more headaches than happiness so thought I would share another approach. It might not exactly be an answer to your question but it might be helpful.

Take one tile you have modeled and add two array modifiers to create horizontal &amp; vertical tiling. Add a lattice modifier in Object mode and adjust it's shape to enclose the roof. In Edit mode you can reshape the lattice to create the drooping shape in the roof.





While not suitable for hyper-realism it is a relatively easy &amp; effective.